Certain peroxidases (lactoperoxidase, myeloperoxidase) exert an antimicrobial effect when combined with H2O2 and an appropriate oxidizable substance such as thiocyanate, iodide, bromide or chloride ions. This antimicrobial system may be operative in milk, saliva and neutrophilic polymorphonuclear leukocytes. We propose to study the properties, mechanism of action and biological significance of the peroxidase-mediated antibacterial systems with particular emphasis on the role of these systems in the neutrophil. The long term goals are 1) to determine the role of these antimicrobial systems in the killing of microorganisms by PMN leukocytes, saliva and milk; 2) to determine whether alterations in the components of these systems might contribute to the pathogenesis of disease and 3) to determine whether the administration of components of these systems might influence the course of disease.